The invention relates to lasers, and more particularly to adjustable mounting units for the optical elements of gas lasers.
Lasers have recently been applied to a large variety of technical areas, such as optical measurement techniques, material processing, medicine, etc.
Due to the special chemical, ablative, spectroscopic or diffractive properties of UV light, there is a big demand for lasers that generate laser beams having a short wavelength in the UV range.
Excimer lasers, such as the ones disclosed in U.S. Pat. Nos. 5,771,258 and 5,438,587, serve well as a laser for generating coherent, high intensity pulsed beams of light in the UV wavelength range.
The excimer lasers described in U.S. Pat. Nos. 5,771,258 and 5,438,587, are pulsed lasers. Pulsing is required in excimer lasers to allow sufficient time between pulses to replace the laser gas within the discharge region with fresh gas and allow the gas used for generating the previous pulse to recover before being used again for another gas discharge. In the discharge region (i.e., discharge gap), which in an excimer laser is typically defined between an elongated high voltage electrode and an elongated ground electrode which are spaced apart from each other, a pulsed high voltage occurs, thereby initializing emissions of photons which form the laser beam.
The laser beam is emitted along the extended ground electrode in a longitudinal direction of the laser tube. To achieve the desired amplification by stimulated emission of radiation, a resonator comprising a reflecting and a partially reflecting optical element disposed at opposite ends of the discharge gap is required. The laser beam leaves the tube through the latter.
If the reflective optical elements are provided outside the gas laser tube, a fully transparent window is provided in alignment with the discharge gap at each end of the tube to seal the tube, as can be seen in U.S. Pat. No. 5,438,587, for example. A mirror or other reflective optical element is then provided in axial alignment with one of the windows and its reflective side facing the window. A partially transparent, partially reflective mirror is positioned outside the tube so that it is aligned with and facing the other window. As a result, the faces of the two reflective optical elements are opposing one another and define a laser light resonator.
If the reflective optical elements are used to seal the tube, the mirror and the partially transparent, partially reflective mirror are integrated into the end walls of the tube at opposite ends of the discharge gap. As a result, no extra windows are required. For lasers emitting light in the ultraviolet range of the electromagnetic spectrum, extra windows have the disadvantage of significantly reducing the efficiency and increasing the operating costs, as the special window materials employed are expensive and deteriorate with use and time and need to be occasionally changed. In addition, the transparent windows closing the tube form extra optical elements resulting in extra losses and reflections on the surfaces. The latter can be removed by inclining the window at Brewster""s angle as taught by U.S. Pat. No. 4,746,201, but invariably the laser output is reduced. Deterioration of the optical elements also cannot be entirely avoided, reducing output and giving rise to the need to replace the rather expensive optical elements after a certain time.
The reflective optical elements that form the resonator must be precisely positioned relative to one another to ensure optimal laser light output power, laser efficiency, and the quality of the laser beam. This is especially true with respect to the angular alignment of the reflective optical elements, not only with respect to each other, but also with respect to the laser tube. However, maintaining the appropriate angular alignment of the reflective optical elements is difficult in view of changes in the operating conditions, such as pressure or temperature of the gas and the temperature of the tube, the optical elements, and their supporting units. In addition, mechanical vibrations or shock to the laser may also affect the angular alignment of the reflective optical elements forming the laser resonator.
When the reflective optical elements are provided outside the laser tube, a very complex outer supporting structure for supporting the reflective optical elements must be provided. Such a supporting structure is very expensive and susceptible to damage. Furthermore, the length of the resonating path between the two opposing mirrors is longer than what is actually necessary. This reduces the output power of the laser, which in turn reduces the efficiency of the laser. In addition, the supporting structure is susceptible to deformation due to outer forces or thermal expansion. Such distortions may distort the angular alignment of the reflective optical elements, particularly the parallelism between the two opposing laser optical elements.
These disadvantages, which are attendant to external supporting structures, have lead to a demand to provide the reflective optical elements as an integral part of the laser tube. However, trying to provide the reflective optical elements as an integral part of the laser tube has caused a different set of problems.
Inside the laser tube, high gas pressures occur, thereby increasing the danger of deformation and damage of the rather sensitive laser optical element. The gas pressure is further increased as a result of the increasing temperature of the gas inside the laser tube caused by the emission of energy. This obviously makes the problem even worse. In addition, thermal expansion of the laser tube can generate a further distortion of the parallel disposition of the laser optical elements with respect to each other.
A mechanism for permitting the reflective laser optical elements to be adjusted with respect to each other is crucial, because light inside the resonator is reflected by the reflective optical elements forming the resonator numerous times. As a result, even a slight divergence from the ideal adjustment may cause a malfunction of the laser or at least a reduction of the laser light output power, and thus reduction in the efficiency of the laser and its beam quality.
A number of patents, including DE 3130399 A1, DBGM 297 15 466.4, U.S. Pat. No. 4,744,091, JP 61-047008, and DE 3710525 C2, teach the use of spacer bars or frames that surround the laser tube and support the reflective optical elements that form the laser""s resonator. These spacer bars or frames also include adjustment mechanisms that permit the reflective optical elements to be adjusted relative to one another and the laser tube. Due to the spacing between the laser tube and these spacer bars or frames, these known mounting structures are not exposed to the operating conditions in the tube. Thus, the operating conditions of the laser do not tend to influence the position of the optical elements. However, such arrangements are difficult to manufacture and service. In addition, they are more prone to distortions resulting from external forces than a support structure for the resonator""s optical elements that are directly coupled to the tube itself.
Thus, a need exists for an improved adjustable mounting unit for mounting the optical elements of a laser.
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
All of the foregoing applications are incorporated by reference as if fully set forth herein.
An object of the present invention is to provide an adjustable mounting unit for an optical element of a gas laser in which it is possible to achieve an improved ability to adjust the position of the optical element while at the same time be able to mount the optical element to the laser tube. The ability to adjust the position of reflective optical elements that define the laser resonator is particularly important to achieving optimal performance from a gas laser.
In order to achieve this first object of the invention, an adjustable mounting unit for an optical element of a gas laser comprising a tube having a first end wall at one end and a second end wall at the other end is provided. The mounting unit comprises a rigid support structure including an aperture, an optical element mounted within the aperture, and at least three adjustable mounting devices to attach the support structure to the laser tube. Preferably the mounting points are selected so that they are displaced in an axial direction by substantially the same amount due to dimensional changes in the laser that occur during operation of the laser. When the adjustable mounting unit is attached to the laser, the rigid support is spaced apart from the end wall of the laser to allow for the adjustment of the angular positioning of the optical element. Furthermore, the aperture and optical element are disposed transverse to the optical axis and are aligned with the optical axis. Adjustment of the adjustable mounting devices changes the angular position of the optical element relative to the optical axis.
In a preferred embodiment of the invention, the adjustable mounting unit further comprises a gas-tight flexible tube element which is used to form a gas-tight seal between the laser tube and the optical element disposed in the aperture of the rigid support structure. Preferably, the flexible tube comprises a base end, an optical element receiving end, an optical element receiving surface within said flexible tube element proximate to the receiving end, and a flexible section interposed between the base end and the receiving surface. The flexible section may comprise, for example, a bellows. The base end of the flexible tube is hermetically attached around the port to the first end wall so that the optical axis of the laser passes through the flexible tube element. The exterior surface of the optical element receiving end is engaged with the aperture wall in the rigid support. Further, the optical element is received by the optical element receiving surface within the flexible tube element and a hermetic seal is formed between the optical element and the optical element receiving surface.
Thus, by employing the flexible tube element as described above, a gas tight chamber may be formed between the end wall and the optical element. This in turn permits the laser to be designed without using a fully transparent lens in the end wall to seal the laser, thereby reducing the number of optical elements through which the laser light must pass.
Preferably the optical element is symmetrically disposed between an even number of the adjustable mounting devices. For example, if the even number of fixation points is two, the center of the optical element preferably falls on a line that bisects the line connecting the two fixation points at their midpoint, and more preferably it is positioned close to the center of the line connecting the two fixation points. The remaining fixation points can be used to tilt the support, and thereby adjust the optical element and the laser unit.
The rigid support structure may comprise a solid plate or an angular structure having a first arm and a second arm that enclose an angle. Preferably the enclosed angle is 90xc2x0.
With the adjustable mounting unit of the present invention, it is now possible to achieve very exact adjustments of the optical elements of a laser, even in a situation where the optical elements are used to provide a gas seal. Furthermore, the adjustable mounting unit of the present invention is very simple and thus cheap. As a result, with the adjustable mounting unit of the present invention, it is no longer necessary to provide a complex outer support mechanism to support a laser optical element outside the laser tube if adjustability is desired.
A second object of the invention is to provide a gas laser with an adjustable mounting unit for an optical element. In particular, it is desired to provide a gas laser with an adjustable mounting unit for an optical element that is capable of being adjusted with improved precision.
To this end, a gas laser is provided that comprises a tube having a first end wall at one end and a second end wall at the other end. The tube defines a cavity for containing a laser gas therein and includes a port in the first end wall. The laser further comprises an optical axis extending longitudinally through the tube and passing through the port. The optical axis is the axis along which the laser light resonates in the laser. A rigid support structure that includes an aperture is mounted to the laser tube so that the optical axis passes through the aperture. An optical element is mounted within the aperture. Preferably the optical element is either a fully reflective or partially reflective, partially transmissive mirror so that it comprises one of the mirrors of the laser resonator. At least three adjustable mounting devices are used to attach the support structure to the laser tube.
To ensure that the laser remains properly adjusted at all times, preferably the mounting points. are selected so that they are displaced in an axial direction by substantially the same amount due to dimensional changes in the laser occurring during the operation of the laser. When attached, the rigid support is spaced apart from the end wall of the laser to allow for the adjustment of the angular positioning of the optical element. Furthermore, the aperture and optical element are disposed transverse to the optical axis and are aligned with the optical axis. Adjustment of one of the three adjustable mounting devices changes the angular position of the optical element relative to the optical axis.
In a preferred embodiment of the invention, the laser further comprises a gas-tight flexible tube element that is used to form a gas-tight seal between the laser tube and the reflective optical element. Preferably, the flexible tube comprises a base end, an optical element receiving end, an optical element receiving surface within said flexible tube element proximate to the receiving end, and a flexible section interposed between the base end and the receiving surface. The flexible section may comprise, for example, a bellows. The base end of the flexible tube is hermetically attached to the first end wall around the port so that the optical axis of the laser passes through the flexible tube element. The exterior surface of the optical element receiving end is engaged with the aperture wall in the rigid support. Further, the optical element is received by the optical element receiving surface within the flexible tube element and a hermetic seal is formed between the optical element and the optical element receiving surface.
Thus, by employing the flexible tube element as described above, a gas tight chamber may be formed between the end wall of the laser and the optical element. This in turn permits the laser to be designed without using a fully transparent lens in the end wall to seal the laser, thereby reducing the number of optical elements through which the laser light must pass.
Preferably the optical element is symmetrically disposed between an even number of the adjustable mounting devices. For example, if the even number of fixation points is two, the center of the optical element preferably falls on a line that bisects the line connecting the two fixation points at their midpoint, and more preferably it is positioned close to the center of the line connecting the two fixation points. The remaining fixation points can be used to tilt the support, and thereby adjust the optical element and the laser unit.
The rigid support structure may comprise, for example, a solid plate or an angular structure having a first arm and a second arm that enclose an angle. If an angular structure is used, preferably the enclosed angle is 90xc2x0. The arms, i.e. the first arm and the second arm, may have different lengths. Preferably, however, the first arm of the rigid support structure is about twice as long as the second arm, and the aperture is formed in the first arm. In a further preferred embodiment of the invention, mounting devices are preferably arranged at the end portions of the arms.
The adjustable mounting devices employed in connection with the present invention preferably each comprise a stud bolt having a first threaded end, a second threaded end, and a body portion interposed between the first threaded end and the second threaded end. The first threaded end is slideably received in a hole in the support structure and extends through the hole. The second threaded end is used to attach the support structure to the laser. An adjusting nut is then threaded onto the first threaded end, and a biasing element is provided to bias the support structure away from the second threaded end of the stud bolt and toward the adjusting nut.
Therefore, with the gas laser of the present invention, it is now possible to achieve very exact adjustments of the optical elements of the laser, even in a situation where the optical elements are used to provide a gas seal. Furthermore, the adjustable mounting unit of the present invention is very simple and thus cheap. As a result, with the adjustable mounting unit of the present invention, it is no longer necessary to provide a complex outer support mechanism to support a laser optical element outside the laser tube if precision adjustability is desired. Furthermore, gas lasers according to the present invention are more efficient because they have a shorter resonance distance. In addition, manufacturing costs are less with gas lasers according to the present invention than a gas laser according to the prior art because the complex supporting mechanisms used in the typical prior art devices are not required with the present invention.
With the present invention it is now also possible to provide a stable support arrangement for the optical elements of a gas laser that is mounted to the laser tube itself and which keeps the optical elements in the appropriate alignment and position independent of the operating conditions of the laser.
The present invention is especially well suited for use in excimer lasers because excimer lasers operate under a very high pressures and temperatures. For example, excimer lasers typically operate at a pressure of about 6 bar and a temperature of up to 100xc2x0 C. or more. With the present invention, however, it is possible to cope with these demanding conditions and provide a precise adjustment of the optical elements of the laser. Furthermore, these adjustments will not become distorted during use of the laser as a result in variations in the operating conditions within the laser tube. Indeed, because the optical element is supported indirectly by the tube edge in the devices according to the present invention, any bending, curving and/or deflecting of the front portion of the laser tube, due to changes in temperature or pressure in the tube, for example, will not affect the alignment of the reflective optical elements mounted in the adjustable mounting structure.
Preferred embodiments of the present invention will now be described in detail in connection with the accompanying drawings.
Other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description of the preferred embodiment taken together with the drawings.